This invention relates generally to a video amplifier for the video display of a television receiver, and particularly relates to a video amplifier having an AGC-clamped feedback loop for level video signal output.
The signal transmitted to and received by a television receiver is termed a composite video signal which includes periodic horizontal and vertical synchronizing pulses for synchronizing television receiver scanning circuits which drive the receiver's kinescope. The synchronizing pulses ensure that the television receiver's scanning circuitry is synchronized with the associated scanning circuitry of the image pick-up device located at the television transmitting station. In the kinescope, the electron beam, or electron beams in the case of a color television receiver, is deflected to produce a horizontal trace line on the kinescope's faceplate. Following completion of a horizontal scan line, the electron beam is rapidly deflected back to the beginning of the next horizontal line and horizontal sweep, or trace, is re-initiated. During horizontal sweep the electron beam is modulated in accordance with video signals provided to the television receiver. The period during which the electron beam is rapidly deflected back to its sweep start position is termed the horizontal re-trace interval. In addition to the electron beam horizontal trace and re-trace intervals, the electron beam is also vertically deflected at a slower rate to permit the electron beam to scan the television receiver's faceplate in a horizontal line-by-horizontal line fashion from the top to the bottom of the screen. Because many horizontal trace lines are used during a single vertical trace interval the receiver's electron beam is horizontally deflected at a much higher rate than its vertical deflection rate.
The composite video signal includes three components: a camera signal incorporating the desired picture information, synchronizing pulses for synchronizing transmitter and receiver scanning, and blanking pulses to make vertical and horizontal retrace invisible. With retrace being initiated by the horizontal or vertical synchronizing pulses, the synchronizing pulses are transmitted as part of the picture signal but are sent during the blanking period when no picture information is transmitted. Part of the modulated picture signal amplitude is used for the synchronizing pulses and the remainder includes the camera signal. The synchronizing pulses are frequently termed sync pulses. The form of the synchronizing pulses is illustrated for the ideal situation in FIG. 1. The synchronizing pulses shown include from left to right three horizontal pulses, a group of six equalizing pulses, a serrated vertical pulse, and six additional equalizing pulses which are followed by three more horizontal pulses. Only three horizontal pulses are shown on each side of the equalizing pulses and serrated vertical pulse for brevity sake, but in practice there are 262.5 horizontal pulses between each vertical pulse. These horizontal pulses represent horizontal trace lines and correspond to one field with the video presentation made up of two interlaced fields. Each field is separated by one wide vertical pulse, which is actually composed of six individual pulses separated by the five serrations. The five serrations are inserted in the vertical pulse at half-line intervals and the equalizing pulses are also spaced at half-line intervals. The equalizing pulses provide identical wave shapes in the separated vertical synchronizing signal for even and odd alternating fields so that constant timing can be obtained for good field interlace. The vertical synchronizing pulse extends over a period equal to six half lines or three complete horizontal lines making it much wider than a horizontal pulse in order to facilitate separation of these synchronizing pulses at the receiver.
Because of the high voltages utilized in the typical television receiver, consumer safety standards dictate that the user be adequately isolated. Thus, double isolation is generally provided in the form of a high voltage sweep transformer and a video transformer. Because of the large bandwidth of the video signal and limitations in the transfer response of the video transformer, a portion of the composite video signal is lost in this stage of video signal processing. In particular, because of the inherent characteristics of the video transformer and size, winding, etc., limitations placed on it in the television receiver environment, a segment of the low frequency portion of the video signal is lost. While this frequency cut-off characteristic of the video transformer does not have an adverse effect on the video signal itself, it does impact synchronization (sync) pulse tip level. The absence of a low frequency component in the video signal causes a shift in the DC level of the sync tips. This signal shift is due to transformer coupling characteristics and the effect of the transformer inductance on signal transfer between transformer windings. The effect of the loss of low frequency components of the video signal on the sync pulse tip levels is shown in FIG. 2. Rather than the level sync pulse amplitude throughout the entire video signal is shown in FIG. 1, there's now a shift in sync pulse tip level due to the loss of lower frequencies from the video signal. The magnitude of the sync tip level shift is designated by an "X" in FIG. 2. This sync tip level shift produces a loss of or inability to achieve vertical synchronization in a television receiver. This results in vertical roll in the video display because of the inability of vertical sync separator circuitry to differentiate between sync pulse level and the reference black level. In older television receivers this undesirable condition could be corrected by manually adjusting vertical synchronization circuitry to lock-on to the vertical sync pulse. However, the increasing use of vertical countdown systems in current production television receivers does not provide for such a manual vertical hold control, thus, in a television receiver utilizing a vertical countdown system it is impossible to manually adjust the vertical control to achieve vertical lock or synchronization.
One approach to providing a video signal with a flat frequency characteristic is disclosed in U.S. Pat. No. 4,184,176 to Sahara et al. This approach employs a feedback amplifier in the video output circuit of a television receiver with the feedback amplifier having two feedback circuits connected in parallel with one feedback circuit negatively feeding back a low frequency component of the output signal from the amplifying circuit to the input of the amplifying circuit. The other feedback circuit negatively feeds back a high frequency component of the output signal. With the two feedback circuits having substantially equal feedback ratios, it is maintained that the phase of the feedback signal can be maintained substantially constant regardless of changes in frequency of the input signal and instabilities in the amplifying circuit, so as to permit the frequency characteristic of the output video signal to be flat to ensure that the kinescope will reproduce a stable picture. While this approach addresses the problem of providing a flat frequency signal characteristic to the kinescope, it does not provide for frequency loss in the video signal caused by video transformer signal transfer characteristics. Another approach to maintaining stable video amplifier operation in a television receiver is disclosed in U.S. Pat. No. 3,970,895 to Willis in which is described a coupled transistor pair forming a luminance signal amplifier. A capacitive coupling device and a second transistor are coupled to the first transistor and form a clamping circuit for maintaining the voltage developed by the first transistor substantially independent of direct current conditions of the source of color difference signals and independent of the voltage applied by the luminance amplifier across the first transistor. One of these amplifiers with capacitively coupled feedback is provided for each of the three color difference signals in providing video signals to the various electron guns in the kinescope. While this approach apparently provides a flat frequency response over a wide video signal bandwidth, it also is not capable of providing frequency compensation for video signal bandwidth distortions caused by video transformer signal transfer characteristics.
These and other problems encountered in the prior art are avoided by the present invention which provides for a stable operating point for a video signal amplifier for the video display of a television receiver by means of an AGC feedback signal in ensuring a level video output signal and in particular a flat vertical sync pulse tip.